Sol-gel method for making ultra-low expansion glass

ABSTRACT

Ultra-low thermal expansion TiO 2  --SiO 2  glasses are prepared using a sol-gel process wherein a stable alkali silicate solution comprising colloidal TiO 2  and having a pH above 9 is gelled to form a semisolid silicate gel, the gel comprising homogeneously dispersed colloidal TiO 2  but being essentially free of agglomerated TiO 2  particles, washing the gel with aqueous media to remove alkali therefrom, and finally drying and consolidating the gel to a clear, void-free TiO 2  --SiO 2  glass which is substantially free of compositional inhomogeneities and has a thermal expansion coefficient below that of pure fused silica.

BACKGROUND OF THE INVENTION

The government has rights in this invention under contract numberF19628-85-C-0048 awarded by the U.S. Air Force.

Fused silica or quartz glasses are well known for certain physicalcharacteristics rendering them unique among glasses. For example, suchglasses demonstrate excellent refractoriness, enabling them to be usedat very high temperatures. They also exhibit chemical inertness,especially to acids. Finally, they possess a very low coefficient ofthermal expansion, i.e., in the range of about 5-10×10⁻⁷ /° C. over thetemperature range of 0-300° C. This latter property of low thermalexpansion renders the glasses particularly valuable for the fabricationof optical components wherein precise dimensions must be retained by thestructure over a rather broad temperature regime.

Fused silica products are presently formed by the fusion of slip-castpreforms, by the fusion of deposits produced through the flame oxidationof silicon-containing source compounds, or by melting silica batchmaterials at very high temperatures, e.g., 2000° C. or above. As can beappreciated, the geometry and dimensions of shapes produced by thesemethods are somewhat limited and, in addition, the physical propertiesof the glasses may vary depending on the source of raw materials. Thusvitreous silica of very high purity is of the highest utility for themanufacture of technical products since properties variations due toimpurities are largely avoided.

The method of choice for the manufacture of high-purity vitreous silicais the flame oxidation of source compounds such as silicontetrachloride. U.S. Pat. No. 2,272,342 provides a general description ofthe manufacture of pure fused silica products by this method. However,as the disclosure of that patent suggests, the shape of products whichcan be formed through this method is quite limited and the cost ofmanufacture for products of complex configuration is therefore quitehigh. Ordinarily, the manufacture of complex structures in pure fusedsilica by this process requires that the glass boules originallydeposited by flame oxidation be cut into plates or other shapes andthereafter fusion-bonded to form more complicated structures.

U.S. Pat. No. 2,326,059 describes a flame oxidation process for glassmanufacture which is closely related to that of the above patent, butwhich produces a glass having an average coefficient of thermalexpansion even lower than that of fused silica. The patent describesdepositing a TiO₂ --SiO₂ glass using a flame oxidation process as forfused silica, but using a mixture of TiCl₄ and SiCl₄ source compounds toprovide a deposited glass wherein the TiO₂ content is about 5-11% byweight. This glass has an average linear coefficient of thermalexpansion of less than about 5×10⁻⁷ /°C., but again, requires cuttingand shaping to provide products of complex shape.

Because of these various fabrication difficulties there is still asubstantial demand for a method for preparing fused silica articles ofhigh purity at relatively low cost and in essentially unlimited shapes.One approach which has been developed to solve this problem, describedin U.S. Pat. No. 3,678,144, involves the so-called sol-gel process. Inthat process, aqueous silicate solutions comprising dissolved alkalisilicate compounds, colloidal silica, and/or quaternary ammoniumsilicate compounds are caused to gel in a controlled fashion to producea semisolid silicate gel which can be further processed to providehigh-silica glass. The process generally involves adding to the silicatesuspension, which is stable at pH values above about 10-11, a gellingagent which is effective to gradually reduce the pH of the solution.This affects a destabilization of the solution and causes theprecipitation of silica therefrom. The silica precipitate forms a silicagel in the liquid medium which can be of very fine and uniform poresize.

The silica gel produced as described can be processed to remove alkalifrom the pore structure if desired, and can thereafter be dried andconsolidated into a dense silica glass product. While obviouslyconsiderable shrinkage is involved in converting the gelled solution tosolid glass, the shrinkage is reproducible and thus products of complexconfiguration can be provided by casting from these solutions.

Concurrently with the development of the sol-gel method described inU.S. Pat. No. 3,678,144, various approaches to the production of glassescontaining oxides other than SiO₂ were proposed. U.S. Pat. No. 3,678,144further teaches that soluble metal compounds dissolved in the silicatesolution may be precipitated with the silica during the gelation processand provide additional components in the resulting porous orconsolidated glasses. However, the metals which may be incorporatedusing that method are largely limited to water soluble, non-volatilecompounds which do not unduly reduce the pH of the silicate solution,and which will form dissolved ionic or complex species in suchsolutions.

An alternative approach to the production of glass products of morecomplex composition is described in U.S. Pat. No. 4,112,032. In thatpatent, particulate additions of oxides or other compounds are made tothe silicate solution, and these additives are trapped in the porestructure of the gel following precipitation and drying to a porousproduct. While TiO₂ is among the particulate materials which may beemployed in accordance with the method of this patent, the method is notdirected to the production of consolidated glasses, and thus nonon-porous high silica glasses are reported.

The desirable low thermal expansion characteristics of TiO₂ --SiO₂glasses such as described in U.S. Pat. No. 2,326,059 are considered torequire a homogeneous glass. That is, the microstructure of the glassmust be such that the TiO₂ is not present in the glass as an isolated orconcentrated titania phase, but is rather uniformly dispersed within thesilica matrix forming the glass on a molecular or atomic scale. Whilesuch homogeneity is readily attained in vapor deposition methods forproducing TiO₂ --SiO₂ glasses, it is difficult to provide the requireddegree of homogeneity in processes involving the physical blending ofthe component solids, or even in silicate solutions.

Attempts to form low-expansion TiO₂ --SiO₂ glasses from solutions ofTiO₂ and SiO₂ in the past have been unsuccessful, due in part to afailure to achieve homogeneous mixed solutions free of precipitatedTiO₂. Silicate solutions comprising TiO₂ in the form of precipitatedcrystallites or larger titania agglomerations will not provide glasseshaving the uniformity of structure required for extremely low thermalexpansion characteristics.

It is therefore a principal objective of the present invention toprovide a method for producing ultra-low expansion TiO₂ --SiO₂ glassesusing a sol-gel process.

It is a further object of the invention to provide a method forproducing ultra-low expansion TiO₂ --SiO₂ glasses which permits themanufacture of products of complex structure by direct casting.

It is a further object of the invention to provide ultra-low expansionTiO₂ --SiO₂ glasses exhibiting average linear coefficients of thermalexpansion (0°-300° C.) below about 5×10⁻⁷ /°C. which include minor buttolerable amounts of alkali metal and iron impurities.

Other objects and advantages of the invention will become apparent fromthe following description thereof.

SUMMARY OF THE INVENTION

The present invention provides a process for making titania-silicaglasses using a sol-gel process which assures a homogeneous glassstructure and therefore provides the ultra-low thermal expansioncharacteristics previously attainable only in vapor-deposited glasses inthis composition system. Whereas aqueous solutions comprising silica andtitania are employed in carrying out the inventive method, the prematureprecipitation of titania encountered in prior attempts to make suchglasses is avoided and thus compositional inhomogeneities which couldundesirably reduce glass clarity, increase the thermal expansioncharacteristics of the glass or, most importantly, result in variationsin glass properties within individual glass components, are notpermitted to develop.

The attainment of homogeneous titania-silica glasses in accordance withthe invention requires the use of aqueous colloidal titania suspensions.Broadly characterized, the method first comprises preparing ahomogeneous aqueous sol wherein the solid phase includes colloidal TiO₂and the liquid phase is a silicate solution comprising dissolved SiO₂compounds. Colloidal SiO₂ may also be present. The pH of the sol ismaintained at a value above that at which either the SiO₂ or the TiO₂will precipitate or gel. Depending on the silicate solution employed, pHvalues in the range of 9-13 are typically maintained.

The silica concentration of the solution or sol is maintained in therange of about 1-12 moles SiO₂ /liter, and the TiO₂ concentration in thesolution is established in the range of about 3-10% of the total of theeffective SiO₂ concentration plus the TiO₂ concentration. By effectiveSiO₂ concentration is meant the concentration of any colloidal SiO₂ plusthe SiO₂ concentration which would result from the conversion of allsilicate compounds in the solution to SiO₂. The titania is preferablyintroduced in the form of an aqueous TiO₂ sol having a pH above about9.0, and is gradually combined with the silicate solution to avoidpremature TiO₂ precipitation.

The homogeneous titania-silica sol or solution thus provided is thencaused to gel, typically by treatment with a suitable gelling agent, toform a homogeneous semisolid gel comprising polymerized SiO₂ andcolloidal TiO₂. This gelation may be effected using organic gellingagents such as have been employed to promote the gelation of alkalisilicate solutions in the prior art.

The gel thus provided is next washed in aqueous media to remove themajority of the alkali metal and/or ammonium ions present therein, thesealkaline components having been introduced by the precursor solutions.Thereafter, the gel is dried to remove water from the pore structurethereof, thus to provide a dried gel consisting primarily of SiO₂ butcontaining the TiO₂ additive homogeneously distributed therein.

Finally, the dry gel is consolidated by heating to a temperaturesufficient to sinter the porous gel to a dense nonporous TiO₂ --SiO₂glass article.

A titania-silica glass product produced in accordance with the abovedescribed method has a composition consisting essentially, in weightpercent, of about 3-10% TiO₂, 90-97% SiO₂, about 10-200 ppm residualalkali metals, and 1-200 ppm residual iron. Notwithstanding the presenceof residual alkali therein, the glass exhibits an average linearcoefficient of thermal expansion over the range 0°-300° C. not exceedingabout 5×10⁻⁷ /°C. This ultralow thermal expansion characteristic of theglass represents a substantial advance in the field of soluble silicatetechnology, particularly in view of the residual alkali content and useof sol-gel processing to make the glass.

The method of the invention is particularly advantageous in theproduction of low expansion silica glass products of complexconfiguration. Although shrinkage of the solution during the process ofgelation, drying, and consolidation can be of the order of 50% by volumeof the original casting, details of the initial cast shape are retainedin the product such that castings of relatively high dimensionalreproducibility and faithful adherence to designed shape may beprovided.

DESCRIPTION OF THE DRAWING

The invention may be further understood by reference to the drawingwherein:

FIG. 1 is a plot showing sample expansion versus temperature for threeTiO₂ --SiO₂ glasses provided in accordance with the invention; and

FIG. 2 consists of electron microprobe plots of TiO₂ content for a priorart glass and for a glass provided according to the invention.

DETAILED DESCRIPTION

Silicate solutions useful for the production of silicate glasses bygelation are known in the prior art. Typically, these are true solutionscomprising silicates, although colloidal suspensions of silica or ofsilicate compounds are also useful. Since silica itself is essentiallyinsoluble in water, the silica in these solutions and/or suspensions isnormally present in the form of a dissolved or suspended alkali metalsilicate or organic ammonium silicate such as a quaternary ammoniumsilicate compound.

In order to maintain the silica in a dissolved or suspended colloidalstate in such solutions, the concentration of alkali metal and/orammonium ion must be sufficient to yield pH values greater than about 10for ammonium silicates, and greater than about 11 for alkali silicates.Further, to secure gelation characteristics suitable for providing afirm homogeneous gel, the silica content of these solutions or colloidsshould be maintained in the range of about 1-12 moles/liter, morepreferably 3-12 moles/liter.

As has been noted above, whereas dispersed titania particles or crystalscan be added to these silicate suspensions and incorporated into the gelstructure as the silica is precipitated therefrom, the resulting gelscannot be consolidated to fully transparent, ultra-low expansion glasseshaving uniform expansion characteristics throughout the body of theglass. In order to achieve the requisite homogeneity for low, uniformexpansion characteristics, the TiO₂ component of the glasses of thepresent invention is incorporated as colloidal TiO₂, most preferably asan aqueous colloidal TiO₂ suspension stabilized at alkaline pH and freeof precipitated TiO₂ particles.

In general, titania sols having pH values in the range of about 9-10 andtitania concentrations in the range of 0.5-3 moles/liter can be used.The preferred sols are those using quaternary ammonium counter ions. Theavoidance of premature titania precipitation from these sols is achievedby maintaining the pH at high values during the step of blending thetitania and silicate components, and by carrying out the blending slowlyand with continued stirring. In cases where the solublizing orstabilizing bases (ammonium or alkali) of the TiO₂ and silicatesuspensions differ, as where a quaternary ammonium titania sol is to becombined with an alkali silicate solution, the preferred practice is tointroduce the silicate slowly into the titania sol, to permit gradualequilibration of the solublizing constituents.

Gelation of the titania-containing silicate solution to form ahomogeneous gel can be induced through the use of gelling agents whichare conventional for the treatment of soluble silicate solutions. Thesegelling agents are compounds which uniformly dissolve in the silicatesolutions, and which react slowly and uniformly therewith to reduce thepH of the solution through the neutralization of alkali and/or ammoniumion present therein. The result is that silica slowly and uniformlyprecipitates from and polymerizes within the liquid phase.

Included among the various compounds suitable for promoting gelation inthese systems are formaldehyde, paraformaldehyde, formamide, glyoxal,methyl formate, methyl acetate, ethyl formate and ethyl acetate. Therate of gelation typically depends upon the amount of gelling agentintroduced into the suspension, as well as on the composition of thegelling agent selected, but useful ranges of addition can readily bedetermined by routine experiment.

A useful alternative to the use of the above-described gelling agents isa self-induced gelation reaction which commences at a very slow ratefollowing the addition of the colloidal titania suspension to thesilicate solution, due to the natural long range instability of the TiO₂sol in strong electrolyte solutions such as potassium silicatesolutions. The rate of this precipitation reaction is sufficiently rapidthat gelation can occur within an interval of 16-24 hours. This issufficient to provide a firm gel of good homogeneity and quality but notso slow as to be commercially impracticable.

Silicate solutions and/or colloidal suspensions which may be used toprovide the solutions to be treated in accordance with the invention maybe formulated of commercially available or other conventional solublesilicate compositions. These compositions are well known, a typicalpotassium silicate solution of the commercially available type generallycomprising about 8.3% K20, 20.8% SiO₂, and the remainder water byweight. A typical colloidal silica suspension will comprise about 40 wt.% SiO₂ with the balance H₂ O.

The gelling of the silicate solutions may be carried out at anytemperature within the range from 0°-100° C., with more rapid gelationoccurring as temperatures above ambient are used to accelerate thepolymerization process. In general, excessive heating of the solutionshould be avoided in order to avoid differential polymerization ratesand/or the generation of gas bubbles or other defects in the evolvinggel. Normally, gelation is completed at the point at which the pH of thesolution is reduced to a value below about 11 for alkali silicatesolutions, or to about 7-10 for ammonium silicate solutions.

The pore sizes in the gelled solutions will vary depending upon factorssuch as the relative proportion of colloidal SiO₂ present in thesolution and the rate of gelling, with larger pore sizes, over thetypical range of about 100-3000 Å, being favored by lower proportions ofcolloidal silica and by slower gelling. For best drying of the gels,pore sizes in the range of about 2000-3000 Å are preferred.

Shrinkage of the gels during the gelling process is substantial and cancause cracking of the gelled shape, especially where the gel tends toadhere to the container. Minimization of this cracking is achieved byusing nonadhering mold materials, such as wax or fluorocarbon plasticcoated molds. Most preferably, the gel is released from the mold as soonas sufficiently gelled to withstand removal, and allowed to completegelation while freestanding in the supernatant liquid.

Following the formation of a silicate gel in the solution, the gel issubjected to a leaching step to remove excess quantities of alkali metalions from the silicate structure. This enhances the purity of the silicaand thereby improves the chemical durability and desirable thermalexpansion characteristics of the product.

Leaching is generally carried out with weakly acidic solutions having pHvalues greater than about 4. Leaching solutions which are conventionalfor the treatment of alkali silicate gels can be used, a typical exampleof such a solution being aqueous ammonium nitrate, at a concentration ofabout 1 molar or below.

Leaching in these gel systems is diffusion limited and the rate ofleaching can be accelerated by increasing the temperature of the system.However, if the pH of the gel is decreased too rapidly, nonuniformleaching and differential condensation of the gel can result, leading toradial cracking of the gel body.

After the gel has been treated for a time sufficient to reduce thealkali level to a sufficiently low value, the gel is dried to removewater or other vehicle constituents and organic reagents from the porestructure thereof. Preliminary drying can be accomplished by simplyallowing the semisolid gel to stand in the ambient environment, althoughforced air drying at 50°-100° C. or microwave heating greatly speedsdrying. Overly rapid drying should be avoided however, as this can leadto the occurrence of cracking in the structure.

After loosely-held water and organic reagents are removed from the bodyby ambient drying, the gel is normally subjected to a further drying orpre-firing treatment to remove bound water and organic constituents.Temperatures above about 1000° C. are suitable for this purpose with apreferred temperature range being about 1000°-1100° C. Heating attemperatures above 1100° C. during pre-firing should not be used, toavoid premature pore closure in the dried gel. Heating times of one houror more at these temperatures is normally sufficient to substantiallyremove bound water and organics from the pore structure.

In carrying out this pre-firing treatment, relatively slow heating ofthe porous body is employed to guard against overly rapid removal ofresidual water from the pore structure. If fast heating is employed,cracking of the porous body can readily occur. The preferred practice isto heat the porous body at a rate not exceeding about 300° C./hr. to thepeak drying temperature employed. Following this treatment, the porousdried gel may be cooled to room temperature and examined for cracks orother defects.

The dried gel produced as described is finally consolidated totransparent, homogeneous glass by further firing. Consolidation can becarried out conveniently at temperatures in the range of about1350°-1700° C. within time periods of 0.5-4 hours, depending upon thesize of the article. As in the case of drying, heating of the article toconsolidation temperatures should be relatively uniform, since differentconsolidation rates can result in shape distortion and/or cracking.

To insure full consolidation and the absence of any entrapped seeds orbubbles in the glass, it may be desirable to carry out the consolidationprocess in an atmosphere of high diffusibility, such as helium. In thisway, complete pore closure is encouraged and the entrapment of residualseeds in the glass is minimized. In some cases, the maintenance ofslightly oxidizing conditions in the consolidation atmosphere may alsobe useful, to avoid undue reduction of Ti⁺⁴ ions to Ti⁺³.

The invention may be further understood by reference to the followingillustrative examples showing the preparation of ultra-low expansionTiO₂ --SiO₂ glasses in accordance therewith.

EXAMPLE I

A homogeneous aqueous alkali silicate solution suitable for thepreparation of a low expansion TiO₂ --SiO₂ glass is first provided. Thesilicate solution is prepared by mixing 180 grams of potassium silicatesolution, 20 grams of a colloidal silica sol, 34 grams of water, and 18grams of formamide, these being mixed and then slowly added to 25 gramsof a colloidal TiO₂ sol with continual stirring. The potassium silicatesolution used is commercially available as Kasil 1 solution from the PQCorporation, while the colloidal silica sol used is commerciallyobtainable as Ludox® HS-40 from E. I. duPont de Nemours of Wilmington,Delaware. The colloidal titania sol is a commercially purchased aqueoustitanate sol available from the Nalco Chemical Co. of Chicago, Ill., astitania sol TX-2588, containing about 14% colloidal TiO₂ by weight andhaving a pH of about 9.6.

The solution thus prepared is very fluid and exhibits no apparent phaseseparation or agglomerated material. It is next split into two equalportions and the first portion is allowed to gel overnight in a closedTeflon® plastic container under ambient conditions with only formamideas the gelling agent. The second portion is gelled rapidly by theaddition, with rapid stirring, of 1.5 cc of ethyl formate, completegelation occurring within an interval of about 1 minute following thisaddition. Condensation of this gel is allowed to proceed overnight, andboth gels are then heated to 80° C. for 2 hours to complete thepolymerization process.

The semisolid gels produced as described are next dealkalized byexposure to a hot aqueous 0.5 N ammonium nitrate solution. The gelsamples are treated by repeated immersion in this solution. Afterammonium nitrate rinsing, the gel samples are next repeatedly rinsed ina heated lN aqueous HCL solution, and are finally rinsed several timeswith hot distilled water.

Following leaching as described, the samples are preliminarily dried forseveral minutes in a microwave oven operating at an average powersetting of about 500 watts, and then slowly heated to 1000° C. in anelectric furnace to remove bound water and residual organic constituentssuch as introduced by the quaternary ammonium cations. These drying andpre-firing steps not only remove all traces of molecular water from thegel, providing a porous glass preform, but also improve the greenstrength of the castings.

The dried gel samples thus provided are finally subjected to aconsolidation heat-treatment in a helium atmosphere. The samples areheated under helium at a rate of about 20° C. per minute toapproximately 1450° C., and maintained at that temperature for about 10minutes to achieve initial consolidation. They are then further heatedin air to 1625° C. and maintained at that temperature for 10 minutes toinsure complete consolidation and densification of the glass.

The consolidated glass samples thus provided are finally cooled to roomtemperature and examined. The products are circular glass wafers,reduced in size from the original castings by about 50 linear percent.The geometry of the original castings is retained, however, and thecastings appear totally transparent with a slight yellow tint. Theyellow color is a consequence of the presence of Fe₂ O₃ impurities inthe silicate starting materials, but the presence of this impurity doesnot effect the thermal expansion and chemical properties of the glass.

The TiO₂ content of these samples is analytically determined to be about7.37% by weight, close to the target composition of 7.4% by weight ascalculated from the solution. The glass has a density of 2.205 g/cc., arefractive index of 1.483, an annealing point of 1040° C., and a strainpoint of 937° C.

EXAMPLE II

A homogeneous aqueous alkali silicate solution suitable for making alow-expansion TiO₂ --SiO₂ glass is provided. This solution is preparedby mixing 220 grams of Kasil 1 alkali silicate solution with 35 grams ofwater, and 20 grams of formamide, and then slowly adding the mixture to25 grams of the aqueous ammonium titanate sol of Example I withcontinuous stirring. The resulting mixture again shows no sign ofsegregation or particulate precipitation, and is stable against gelationfor a period of at least several hours.

Employing the gelling procedure reported in Example I, the silicatesolution is split into two portions, with the first portion beingpermitted to gel overnight at ambient temperatures in an enclosedplastic container. The second casting is rapidly gelled by the additionof 1.6 cc of ethylformate, with complete gelation again occurring withinan interval of about 1 minute. Further gel condensation occurs bystanding overnight, and both samples are heated to 80° C. for 2 hours toensure completion of the polymerization.

The semisolid gel samples thus provided are dealkalized andpreliminarily dried as described in Example I. Thereafter, the porousdried gel samples are consolidated at 1450° C. under helium, followed bya 10 minute exposure to air at 1625° C.

The glass articles thus provided are finally cooled to room temperatureand examined. Again, the glass samples are clear with a slightly yellowtint, showing no evidence of phase separation or retained porosity orseeds. The TiO₂ content of the samples is analytically determined to beabout 7.4% by weight.

EXAMPLES III-VII

Several additional aqueous alkali silicate solutions suitable forpreparing low expansion titania silica glasses are provided. Thesolutions contain varying proportions of potassium silicate solution andcolloidal TiO₂ sol in order to produce titania-silica glasses over arange of composition in the region of low thermal expansion TiO₂ --SiO₂glasses.

Table I below reports compositions for 6 silicate solutions prepared forthis purpose. Included in Table I for each of the 6 examples shown arethe concentration of potassium silicate solution present in thesolution, the amount of water added, the concentration of the formamidegelling agent used, and the amount of colloidal TiO₂ sol (14% weightTiO₂) added to achieve the desired TiO₂ concentration in the glass. Alsoincluded for each of the solution compositions is the nominal or targetTiO₂ concentration for the glass product to be produced using thedesignated solution.

                  TABLE I                                                         ______________________________________                                               Example No.                                                                   3     4       5       6     7     8                                    ______________________________________                                        Potassium                                                                              110     110     110   110   110   110                                Silicate                                                                      (grams)                                                                       H.sub.2 O (grams)                                                                      20      19      18    17    16    15                                 Formamide                                                                              10      10      10    10    10    10                                 (grams)                                                                       TiO.sub.2 Sol                                                                          10      11      12    13    14    15                                 (grams)                                                                       Nominal TiO.sub.2                                                                      5.98    6.54    7.09  7.64  8.20  8.71                               Content                                                                       (% weight)                                                                    ______________________________________                                    

Each of the representative compositions reported in Table I forms arelatively stable solution substantially free of precipitated TiO₂, andeach can be converted to a homogeneous TiO₂ --SiO₂ gel by treatment withone or more of the gelling agents hereinabove described.

Gels produced from the solutions shown in Table I above can be leachedand preliminarily dried, prefired to remove bound water and organics,and then consolidated to dense, nonporous glass of approximately thetarget TiO₂ --SiO₂ composition shown in Table I.

Table II below reports the results of such processing for the glasscompositions reported in Table I. Included in Table II for selectedcompositions are the nominal (Nom.) and in some cases the analyzed(Anal.) TiO₂ content, in weight percent TiO₂, the appearance of theconsolidated glasses, and thermal expansion data for glass samples ofthe compositions described, where determined on individual samples,reported as average coefficient of thermal expansion (C.T.E.) values(×10⁻⁷ /°C.) over the temperature range of approximately 0-300° C. Inall instances, C.T.E. values are reported for glass consolidated at1625° C. Some of the samples were refired to 1675° C. to achieve furtherhomogeneity, with lower expansion values generally being observed afterrefiring at the higher consolidation temperatures.

                  TABLE II                                                        ______________________________________                                               Ex. No.                                                                       3     4       5       6     7     8                                    ______________________________________                                        TiO.sub.2 (Nom.)                                                                       5.98    6.54    7.09  7.64  8.20  8.71                               (% wt.)                                                                       TiO.sub.2 (Anal.)                                                                      --      6.40    --    --    8.08  8.64                               (% wt.)                                                                       Glass    clear,  clear,  clear,                                                                              clear,                                                                              clear,                                                                              clear,                             Appearance                                                                             yellow  yellow  yellow                                                                              yellow                                                                              yellow                                                                              yellow                                      tint    tint    tint  tint  tint  tint                               C.T.E.   4.0     3.3     2.6   1.9   1.0   0.6                                (× 10.sup.-7)                                                           (Consolidated                                                                 at 1625° C.)                                                           C.T.E.   --      2.2     0.9   --    --    0.1                                (× 10.sup.-7)                                                           (Consolidated                                                                 at 1675° C.)                                                           ______________________________________                                    

As the data in Table II indicate, clear TiO₂ --SiO₂ glasses can beprepared in accordance with the invention over a relatively broad rangeof TiO₂ --SiO₂ composition, and homogeneous glasses having expansioncoefficients below that of fused silica glass can readily be provided.Moreover, those glasses generally have optical and thermal expansionproperties which are essentially constant over the entire dimensions ofthe glass products produced.

While glasses provided in accordance with the inventioncharacteristically comprise 1-200 ppm of iron and 10-200 ppm of residualalkali metals, the preferred glasses will nevertheless have averageexpansion coefficients very near zero over the 0°-300° C. range.Particularly preferred from the standpoint of low thermal expansion areglasses containing about 7-10% TiO₂, especially when consolidated withinthe relatively high consolidation range of 1650°-1700° C.

The low expansion characteristics of glasses provided in accordance withthe invention are further illustrated in FIG. 1 of the drawing, whichplots sample expansion (ΔL) as a proportion of sample length L, in partsper million, over the temperature range of about 25°-300° C. for threeof the inventive glasses. The lowest expansion is demonstrated by aglass containing about 8.71% TiO₂.

The homogeneity of glasses provided in accordance with the invention isshown in FIG. 2 of the drawing, which compares surface electronmicroprobe plots of TiO₂ content for a glass of the invention (NovelGlass) and a commercial TiO₂ --SiO₂ glass sold as Corning Code 7971glass (Prior Art Glass). In those plots the TiO₂ content, in weightpercent, is shown on the vertical axis and the position on the samplesurface is indicated on the horizontal axis, from an arbitrary startingpoint across a dimension of 300 microns on the sample surface. Thehomogeneity of the inventive glass is judged to be substantiallyequivalent to that of the commercial vapor-deposited glass whenevaluated by this method.

I claim:
 1. A method for making a low expansion TiO₂ --SiO₂ glass whichcomprises the steps of:(a) preparing an aqueous sol wherein the liquidphase comprises at least one soluble silicate compound selected from thegroup of alkali silicates and ammonium silicates, and wherein the solidphase comprises colloidal TiO₂, the sol being formed by combining a TiO₂sol with a silicate solution containing dissolved SiO₂ compounds, saidaqueous sol having a pH of at least 9 and being substantially free ofprecipitated SiO₂ and TiO₂, the proportion of TiO₂ in the sol being suchas to provide a TiO₂ concentration of TiO₂ and SiO₂ ; (b) gelling thesol to form a semisolid silicate gelled shape wherein the colloidal TiO₂is homogeneously distributed; (c) removing alkaline constituents fromthe semisolid gelled shape to provide a dealkalized gel; (d) drying thedealkalized gel to remove water and residual volatile solutioncomponents and to provide a porous body therefrom; and (e) firing theporous body to a temperature sufficient to consolidate it to anon-porous TiO₂ --SiO₂ glass article.
 2. A method in accordance withclaim 1 wherein the sol has an effective silica concentration in therange of about 1-12 moles of SiO₂ per liter.
 3. A method in accordancewith claim 2 wherein the soluble silicate compound is an alkali silicatecompound, and wherein the sol has an effective SiO₂ concentration of3-12 moles per liter.
 4. A method in accordance with claim 3 wherein thesolid phase of the sol additionally comprises colloidal SiO₂.
 5. Amethod in accordance with claim 3 wherein the sol is gelled by theaddition of a gelling agent thereto which is effective to reduce the pHof the sol to a value at which SiO₂ will precipitate therefrom as asemisolid gel.
 6. A method in accordance with claim 5 wherein thegelling agent is selected from the group consisting of formaldehyde,paraformaldehyde, formamide, glyoxal, methyl formate, methyl acetate,ethyl formate, and ethyl acetate.
 7. A method in accordance with claim 3wherein the TiO₂ is provided as an aqueous colloidal TiO₂ suspensioncontaining quaternary ammonium ions and having a pH above about
 9. 8. Amethod in accordance with claim 7 wherein the colloidal TiO₂ in the solis present in a concentration providing a TiO₂ content in the range ofabout 7-10% of the total effective concentration of TiO₂ and SiO₂ in thesol.
 9. A method in accordance with claim 8 wherein the gel has a poresize of 2000-3000 Å.
 10. A method in accordance with claim 2 wherein thestep of removing alkaline constituents from the semisolid gel comprisesthe immersion of the semisolid gel in an aqueous acidic medium.
 11. Amethod in accordance with claim 2 wherein the step of drying thedealkalized gel comprises heating the gel to a temperature in the rangeof about 1000°-1100° C. to volatilize water and organic constituentstherefrom.
 12. A method in accordance with claim 2 wherein the step ofdrying comprises microwave heating.
 13. A method in accordance withclaim 2 wherein the step of firing the gel comprises heating the gel toa temperature in the range of about 1350°-1700° C. to consolidate thegel to a dense, non-porous glass.